Chen Hao (Kali Power Supply Bureau of Guizhou Province, Kaili 556000, Guizhou, China) focuses on the analysis and study of the laying, installation and handover testing of power cables, and expounds the application of cables in production.
In the construction of cities and townships in China, the construction of distribution networks is a crucial link. Effectively improving the construction quality of power cable projects is an important factor in the safe and reliable operation of power grids.
1 Laying of cables The laying of cables includes direct laying, laying of cable trenches, laying of pipes, laying of cable trays, and laying of concrete pipes. The method of laying is as follows: First of all, it must be checked whether the laid cable is wet or not, whether it is damaged by the machine, and if the cable is in these two situations, it cannot be laid. Check whether the various tools and materials required for laying are qualified. After the preparation is completed, the power cable can be transported to the job site for laying.
Now take the buried cable laying equipment as an example: According to the design drawings, site conditions determine the direction of the power cable, measuring the length of the cable path, and then intercept the power cable according to the length of the path, usually more than 12m easy to lay; then The location of the cable trench and the width of the excavation are marked on the ground with white lime. The width is determined according to the width of the human body and the number of cables. Usually the width of the excavation is 0.50.6m, the depth is 0.8m, and the trench should be vertical Excavation, excavated mud piled on both sides of the ditch for easy backfilling. After the cable trench has passed acceptance, a 10cm thick layer of sand is laid on the bottom to protect the cable. Finally, use mechanical gravitation or manpower for cable laying and put the cable in the ditch. After passing the inspection, cover a 10cm layer of soft soil and fire sand around the cable, cover the protective cover, and backfill the ditch on both sides of the ditch. Inside, laying ends.
2 Installation of power cables When installing power cables, ensure that the ambient temperature and the temperature of the power cables themselves exceed C, and that the installation can be performed only after 24 hours. Especially PVC plastic, it will become harder and harder as the ambient temperature decreases, more and more brittle. If it is subjected to violent impact below 0C, there is a danger of PVC plastic breaking.
According to the length of the power cable bracket to the electrical connector, the plastic sheath is cut off. At a distance of 3cm from the opening, saw the armor plate. Use a plastic bag to fasten the armor. Cut the inner sheath 1cm away from the armor. Then use a blowtorch to heat the anticorrosive coating, and then clean the coating with gauze. Put in PVC gloves, seal the lower mouth of the glove with heat-shrink tubing, leave a 2cm shielding layer on the glove's mouth, loosen the semi-conductive fabric, and use the self-adhesive rubber tape to separate the semi-conductive layer from the nozzle. A stress hammer is packaged at 0.5 cm and a rubber band is wrapped on the stress hammer to seal the finger. Mark the phase sequence and put a yellow-green-red thermal plastic tube on the three-phase heart. A rain cover is placed on the outer end of the cable core and fixed with a self-adhesive rubber band. The indoor cable can save this item. According to the increase in the depth of the line nose 0.5cm stripped after the core insulation, put on the line nose, using air pressure clamp pressure, the cable is installed to the electrical connection. If necessary, put a protective tube on the power cable to prevent corrosion and mechanical damage.
The following points need attention in the installation of power cables: (D) Before construction, check whether the cable is damp. The specific method is to ignite the cable insulation paper or put it into the cable oil around 150C for inspection.
The power cable must not be turned sharply. According to the standard of GB/T127062002, the laying process must be strictly in accordance with the standards, and a large bending radius should be used as far as possible during the laying process. Control cables, plastic power cables, rubber insulation and plastic sheathed power cables must not be smaller than 10D (D is the outer diameter of the cable) Oil-impregnated paper Absolute power cable, rubber insulation, bare lead-wire power cable is not less than 15D, rubber insulated lead package Armored power cables must not be less than 20D. When burying power cables in cold areas, the power cables should be buried below the frozen ground.
When the power cable passes through various sections subjected to pressure and vibration, the construction shall be protected by pipe. These include: foundations, floors, and walls of buildings; roads and roads that may be subject to mechanical damage; cables that are 2m from the ground to 0.2m below the ground are prone to contact with pedestrians and may be subject to mechanical damage.
When the power cable is laid parallel to the building, the cable must be buried outside the building's bulk water. When entering the building, the protection pipe worn must exceed 10cm outside the building's bulk water. When the buried power cable crosses the railway and the road, the protection pipe worn should extend 1m. When the power cable crosses the heat pipe, such as The cable shall be protected by an asbestos cement pipe and the length shall extend 2m on both sides of the thermal pipe groove, and shall be protected by a heat insulation layer. The heat pipe trench and each side of the cable shall be over 1m. When the power cable is laid underground, the connector box shall be underneath. The concrete foundation board must be laid, and the length should extend 0.60.7m from both sides of the joint protection box. 3 Handover test of power cable project Doing a good job of handing over the power cable test is an important task to ensure safe operation of the power grid. It is stipulated that a DC voltage withstand test should be used. As the DC withstand voltage test is performed, electrons may be injected into the crosslinked cable dielectric polymer to accumulate the space charge at the convex or tiny voids of the semiconductive layer. After the cable is put into operation and the AC voltage is applied, Superimposed on each other, insulation breakdown occurs.
After the DC voltage withstand test, there will still be a lot of residual charge in the power cable. In order not to affect the measurement of the insulation resistance and the absorption ratio, the cable should be fully discharged in time. The discharge time of the cable after the first DC withstand voltage test is short. If the insulation resistance test is carried out without exhausting the remaining charge, both the sink current and the charge current will be reduced compared to the first time, resulting in insulation. The resistance increases falsely and the absorption ratio decreases.
According to domestic and foreign experimental studies, there are obvious shortcomings in the DC voltage withstand test: In the DC voltage, the electric field distribution of the cable insulation and the electric field distribution under the AC voltage are different, which does not reflect the actual operating conditions.
The space charge formed by the DC voltage during the test will lead to surface flashover and breakdown during the use of the cable.
The ever-improving test equipment makes AC withstand voltage test possible. The 30300Hz frequency withstand voltage test voltage value of the power cable is shown in Table 1.
It should be noted that the above data is the test data of rubber cables. The test data of paper-insulated cables and rubber-plastic cables are different. It is necessary to judge according to the construction environment and use rubber-plastic cables as the test for paper-insulated cables.
4 Development trend of cable and its application in production 4.1 Construction of large-scale power grids and promotion of industry development During the “Eleventh Five-Year Plan†period, China’s investment in power grids amounted to 1.2 trillion yuan, and the “Tenth Five-year Planâ€.
During the period, it increased 2.4 times. The extensive application of cables in power grid construction has also ushered in a new round of rapid development in this industry.
During the “Eleventh Five-Year Plan†period, the deformation characteristics of the new 330kV (turning to page 136) specimens in China are elastic deformation first, followed by uniform plastic deformation, necking occurs after plastic deformation reaches a certain level, and plasticity occurs locally. Deformation, the final fracture, fracture occurred near the fusion line of the joint. The tensile mechanical tests of TIG welding and conventional TIG welding joints show that the tensile strength of A-TIG welding specimen is about 590MPa, the tensile strength of conventional TIG welding specimen is about 560MPa, and the resistance of A-TIG welding specimen is Tensile strength is about 5% higher than that of conventional TIG welding specimens, elongation is increased by about 27%, and both strength and plasticity are increased.
2.2A-TIG Weld Stress Corrosion Cracking Enhancing Factors There are three factors affecting stress corrosion cracking. These three elements can be summarized as follows: Stress corrosion cracking can only be caused by tensile stress. This tensile stress can be the stress caused by the applied load; it can also be a variety of residual stresses, such as welding residual stress, heat treatment residual stress and assembly stress.
The environment in which stress corrosion occurs is always a corrosive medium. This corrosive medium is generally weak. If there is no simultaneous effect of tensile stress, the material will corrode very slowly in various media. The stress corrosion-causing medium is specific, that is, each material is sensitive to only one medium, and this medium may have no significant effect on other materials. Austenitic stainless steels are susceptible to stress corrosion in corrosive environments containing chloride ions.
Generally, only the alloy produces stress corrosion, and pure metal does not produce this phenomenon. Alloys can only produce stress corrosion cracking when the tensile stress is combined with specific corrosion media.
The differences in stress corrosion cracking resistance between A-TIG welding specimens and ordinary TIG welding specimens were analyzed from the above three factors. In the 41% magnesium chloride stress corrosion test method, the A-TIG welding sample and the ordinary TIG welding sample are in two groups and are in the same etching solution. The two materials are the same and the corrosive medium is the same, so the stress corrosion cracking resistance is The difference is mainly due to the difference in tensile stress. In the process of stress corrosion test sample processing, external tensile stress is applied, but the sample is processed strictly in accordance with national standards, and the degree of bending and clamping are basically the same. Therefore, the influence of the difference in welding residual stress is mainly considered in the analysis process. .
According to Ye Song's research, conventional TIG specimens have a greater cracking rate under greater tensile stress, and their stress corrosion cracking resistance is weaker. The appearance time of A-TIG weld cracking is later than that of conventional TIG welding, and the depth of crack propagation is lower than that of conventional TIG welding. It shows that the welding of the active agent improves the stress corrosion cracking resistance of the workpiece to some extent.
During the formation and propagation of stress corrosion cracking, due to the greater tensile stress in the overheat zone of conventional TIG welding specimens, the formation of cracks is earlier and the expansion speed is faster, so cracks quickly penetrate the hot zone metal and undergo brittle fracture.
3 Conclusion Active soldering is a special TIG soldering method that adds active agent in the welding process. Its outstanding advantage is that it can significantly increase the penetration depth of the weld and reduce the impermeability and other defects.
Stress corrosion studies show that under the same test conditions, the crack initiation time of A-TIG welding samples is later than that of conventional TIG welding, and the penetration depth of cracks is smaller than that of conventional TIG welding samples. The integrated quality of A-TIG after welding is obviously better than that of conventional TIG welding.
Liu Fengyu. Study on the Mechanism of Welding and Melting Increase of Stainless Steel and Alloy Active Agents .
Harbin Institute of Technology, 200(1): 17 Zhang Ruihua, Fan Ding, Yin Yan. Study on Active Agents for Low Carbon Steel High-efficiency TIG Welding. Gansu Gong Yang Chunli, Lin Sanbao. Arc Welding Foundation Harbin Institute of Technology Press, welding and assembly of force equipment.
(Continued from page 129.) Table 1 30300Hz frequency withstand voltage test voltage value of power cable Rated voltage voltage transfer test multiple of the cable CUo) The above AC line is about 36,000 km, and the capacity for power transformation is 180 million kVA. According to the annual 7000km Speed ​​growth, capacity increase has also increased 3 4.2 Optimize large-scale enterprises, accelerate the pace of development At present, the concentration of the cable industry is not high, so the integration of industry resources is imminent, smaller companies will gradually be eliminated from the market. Once the concentration of the industry increases, large-scale high-quality enterprises will certainly develop rapidly and become the industry leader.
Under this trend, the scope of application of the cable will be wider and the demand will be greater.
Rubber cable is one of the most common cables in daily life. It is used in various electrical equipment (such as electric machinery, household appliances, etc.), and it can be adapted to various outdoor environments and is extremely durable. Rubber cable has a complete series of specifications, strong versatility, and stable performance.
Whether it is the northern or southern climatic conditions, how bad the outdoor temperature, rubber and plastic cable can handle with its excellent durability. Rubber and plastic materials also greatly increase the service life of cables, which reduces the probability of accidents caused by cable damage and saves money for multiple maintenance and repair of cables. Therefore, rubber and plastic cables have become the 5th phrase in cable production. With the continuous development of society, the process of city and township construction is also accelerating.
In this process, the distribution network is an essential construction project. How to effectively improve the construction quality of power cable projects has become an important factor in improving the safety of power grids and ensuring their reliable operation. Therefore, it is very necessary to study the laying, installation and handover tests of power equipment cables. Only under the guidance of scientific theories and the experience of previous generations, and in light of the actual conditions, we will be able to flexibly grasp and apply technologies in practice and continuously improve the level so as to better serve socialist construction.
Xia Xinmin. Power cable selection and laying. Chemical Industry Press, 2001.5 GB50150-1991 Electrical installation engineering. Test standards for hand-over of electrical equipment. DL/T5961996 Preventive test procedures for electrical equipment. Power cable laying method selection. China Electric Power Corporation Management, 2009 Li Shijun. Power Engineering Cable Quality Control Technology . China Construction Information, 2007 (22) Electrical Testing.
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